Hot grain dynamics by electric charging and magnetic trapping in debris disks

Abstract The recent discovery of hotdust grains in the vicinityof main-sequencestars has become a hotissue among the scientific community of debris disks. Hotgrains must have been enormously accumulated near their sublimationzones, but it is a mystery how such a high concentration of hotgrains is sustained. The most difficult conundrum is that the size of hotdust grains is estimated to lie in the submicrometer range, while submicrometer-sized grains are instantly swept away from near- stellarenvironments by stellar radiation pressure. One and only mechanism proposed for prolonging the residence time of hotgrains in the near- stellarenvironments is trapping of charged nanoparticles by stellar magnetic fields. We revisit the model of magnetic grain trapping around main-sequencestars of various spectral classes by taking into account sublimationand electric charging of the grains. The model of magnetic grain trapping predicts that hotdust grains are present in the vicinityof main-sequencestars with high rotation velocities and intermediate magnetic-field strengths. On the contrary, we find that the detection of hotdust grains has no correlation with the rotation velocities of central stars nor the magnetic field strengths of the stars. Our numerical evaluation of electric grain charging indicates that the surface potential of submicrometer-sized grains in the vicinityof main-sequencestars is typically 4– 5 V , which is one order of magnitude smaller than the value assumed by the model of magnetic grain trapping. On the basis of our numerical simulation on sublimationof dust grains in the vicinityof a star, it turns out that their lives end due to sublimationin a timescale much shorter than the period of one revolution at the gyroradius. It is, therefore, infeasible to dynamically extend the dwell time of hotgrains inside the sublimationzone by magnetic trapping, while we cannot completely rule out the possibility of magnetic grain trapping outside the sublimationzone where the strength of stellar magnetic fieldhas been underestimated in the previous model. Nevertheless, the independence of hotdust detection on the stellar rotationalvelocity and magnetic field strength favors a scenario that some other (yet unnoticed/overlooked) ubiquitous mechanism of grain trapping is at work.